ZERON 100 FGD Plants

REPORT

The Use of ZERON 100 In Flue Gas Desulphurisation Plant

Prepared by: Roger Francis Corrosion Services Manager

Approved by: Geoff Warburton Product Manager

CIRCULATION Division Technical Job No. Reference No. Report No: TN780 Iss No. 6 Date: November 2008

Copyright © Rolled Alloys 2008 Rolled Alloys Company Confidential Ltd co. registered in USA Rolled Alloys is the owner of the Copyright in this (Delaware)-#37-1540008, PO Box 1287, document. The document and its text, images, diagrams, Northbrook, Illinois data and information it contains must not be copied or 60065. UK Company reproduced in whole or in part, in any form or by any Number FC027795 VAT General Release Y Reg No. GB 803 8704 36 means, without the prior written consent of Rolled Alloys

THE USE OF ZERON 100 IN FLUE GAS DESULPHURISATION PLANT

TABLE OF CONTENTS

SECTION DESCRIPTION

SUMMARY

1.0 INTRODUCTION

2.0 ZERON 100

3.0 CORROSION

3.1 General Corrosion

3.2 Crevice Corrosion

3.3 Erosion Corrosion

4.0 APPLICATIONS AND EXPERIENCES

5.0 CONCLUSIONS

REFERENCES

Table 1. The nominal composition of some common stainless steels.

Table 2. Minimum mechanical properties of some stainless steels.

Table 3. Critical crevice temperature for some stainless steels in a simulated anthracite FGD slurry at pH4.

Table 4. Critical crevice temperature for some stainless steels in a simulated lignite FGD slurry at pH4.

Figure 1. Schematic diagram of a typical FGD absorber tower (Babcock design).

Figure 2. Iso-corrosion curves (0.1mm/y) for some stainless steels in sulphuric acid.

Figure 3. Iso-corrosion curves (0.1mm/y) for some stainless steels in hydrochloric acid.

Figure 4. Iso-corrosion curves (0.1mm/y) for some stainless steels in sulphuric acid plus 2g/L chloride.

Figure 5. Corrosion of some nickel alloys and Zeron 100 in ASTM G28A test.

Figure 6. Pin erosion rig tests results in a simulated FGD slurry.

SUMMARY

The report highlights the corrosion data generated in simulated flue gas desulphurisation (FGD) environments for ZERON 100, the potential applications, and some of the uses in the UK.

1.0 INTRODUCTION

Flue Gas Desulphurisation (FGD) is used to remove sulphur containing gases (mostly sulphur dioxide) from the flue gas discharges of coal fired power stations. The most common method for doing this is the wet limestone process, where a slurry of crushed limestone and water is passed down an absorber tower as the flue gases rise up. The limestone is converted to gypsum (calcium sulphate) which is periodically removed and fresh limestone is added. Because of the recirculation method and the wish to minimise water additions, the dissolved solids content of the slurry is often high, depending on the nature of the coal being burnt and the composition of the make up water. Chlorides, in particular, can be very high; 40,000mg/l is not uncommon, but the sulphate content is also high. During operation the temperature can be in the range 450 to 700C and the pH is generally from 4 to 6, depending on the type of coal being burnt.

Figure 1 shows a schematic diagram of a typical FGD absorber tower. This shows some of the areas where either corrosion resistant alloy components or rubber lined steel are used e.g. absorber tower walls, slurry recirculation pumps.

There are two principal types of coal burned in power stations and these give rise to somewhat different slurry compositions.

Anthracite or hard coal is generally low in fluorides, but, at least in the UK, the chloride level is high. This gives rise to a slurry with the following characteristics :

Chloride - 20 to 40 g/l Fluoride - ~ 50 mg/l Temperature - 450 to 500C pH - 4.0 - 5.5

Lignite or brown coal is generally higher in fluorides, although most of these will be insoluble CaF2, and only soluble fluorides will influence corrosion. Chlorides in lignite fuels are lower than with anthracite, while operating temperatures are higher. Typical lignite slurry characteristics are as follows :

Chloride - 5 to 15 g/l Fluoride - 100 to 200 mg/l Temperature - 600 to 700C pH - 4.5 to 6

When sulphur dioxide is oxidised to gypsum the overall reaction is :

2 SO2 + O2 + 2 Ca (HCO3)2 CaSO4 + 4CO2 + 2H2O

However, the oxidation of sulphur dioxide does not occur in a single step and there are a number of intermediate stages. These result in the formation of partially oxidised sulphur 2- 2- 2- species such as sulphite, (SO3 ) thiosulphate (S2O3 ), dithionate (S2O6 ) and possibly others.

Sulphites react rapidly with oxygen to form sulphate, and are generally thought to have no significant effect on the corrosion of stainless steels in aerated solutions. Thiosulphate has been shown to affect the resistance of 304 and 316 stainless steels to pitting at elevated temperatures in acid brines (1). There is nothing published on the effect of dithionate on the pitting of duplex stainless steels.

In addition to partially oxidised sulphur species, real FGD slurries also contain significant quantities of metal ions such as Fe3+, Al3+ and Mn2+. There are also halides present such as the fluorides and chlorides discussed above and bromides as well. All of these species can affect the corrosion behaviour and it is important that they are present in laboratory test slurries.

2.0 ZERON 100

Zeron 100 is a superduplex stainless steel with a composition producing a microstructure which is 50% ferrite and 50% austenite. This combines the properties of strength and ductility. Zeron 100 is available in both cast and wrought product forms, and the nominal compositions are shown in Table 1, along with those of some other common stainless steels.

The high levels of chromium, molybdenum and nitrogen in the alloy give it excellent resistance to localised corrosion in chloride containing fluids. The additions of copper and tungsten give the alloy resistance to acids, particularly mineral acids such as sulphuric and hydrochloric acids.

An indication of the resistance to pitting in chloride solutions is given by the Pitting Resistance Equivalent Number (PREN) which is an empirical relationship linking the chromium, molybdenum and nitrogen contents of corrosion resistant alloys.

PREN = % Cr + 3.3% Mo + 16% N

Common austenitic alloys such as 316L have a PREN of 24, while 22%Cr duplex has a PREN of 34. Zeron 100 has a guaranteed minimum PREN of 40, and is the only alloy to guarantee a minimum PREN value.

The minimum mechanical properties of Zeron 100 at room temperature are shown in Table 2, with those of austenitic alloys such as 316L and 6%Mo austenitic for comparison. It can be seen that the proof stress of Zeron 100 is more than 2.5 times that of 316L and 1.8 times that of 6%Mo. The high strength means that substantial reductions in wall thickness can be obtained, particularly in high temperature / high pressure applications. In addition to savings in the cost of parent material, this also results in savings in fabrication costs and time.

The useful operating range for Zeron 100 is the same as for other duplex and superduplex alloys i.e. -600C to +3000C. At temperatures lower than -600C there is a decrease in impact toughness. At temperatures above 3000C the alloy slowly becomes embrittled due to the precipitation of a third phase (alpha prime).

Zeron 100 is readily welded, by all the common arc processes i.e. TIG (GTAW), MMA (SMAW), SAW etc. For as-welded use it is fabricated with overalloyed Zeron 100X consumables, which have a higher nickel content to ensure the correct phase balance in the weld metal. Material thicknesses from 1.6mm to 63mm have been welded successfully and it is estimated that over one million welds are now in service around the world. In common with other high alloy materials, successful welds in Zeron 100 require qualified welders working to approved procedures.

3.0 CORROSION

3.1 General Corrosion

In certain parts of FGD plant hot, strong acids can condense. Zeron 100 has excellent resistance to mineral and organic acids. Figures 2 and 3 shows the iso-corrosion curves (0.1mm/y) in sulphuric and hydrochloric acids respectively. The results clearly show the superior performance of Zeron 100 compared not only with 6%Mo austenitics, but also with other superduplex alloys such as UNS S32750. This is believed to be due to the copper and tungsten content of the alloy, in addition to the chromium and molybdenum. Copper and tungsten are not present in other alloys, such as S32750.

In many acid systems chlorides can also be present, and the unique composition of Zeron 100 gives it excellent resistance to such fluids. Figure 4 shows the iso-corrosion curves (0.1mm/y) for Zeron 100 and some competitor alloys. The superior performance of Zeron 100 is clearly demonstrated.

The ASTM G28 test is often used to rank materials for service in aggressive oxidising acidic environments. Method A (boiling 50% H2SO4 + 42g/l ferric sulphate) is used for welded samples and Figure 5 shows the corrosion rates for welded Zeron 100 against three commonly specified nickel alloys. It can be seen that Zeron 100 has a much lower corrosion rate than all the nickel alloys commonly considered for these aggressive environments.

3.2 Crevice Corrosion

There is obvious concern that alloys for use in FGD scrubber systems may suffer from crevice corrosion, as crevices abound in all commercial designs. Early laboratory tests (2) were conducted in simple sulphate/ chloride solutions and the results suggested that pH had little effect on the critical crevice corrosion temperature (CCT). However, more recent work by Weir Materials & Foundries (3) in more realistic slurries has shown that pH has a strong effect and the CCT can decrease over 200C as the pH decreases from pH 5 to pH 3.

During normal operation pH levels are generally from 4.5 to 6, but under adverse conditions pH values as low as 4 are possible. Hence WM&F has carried out a series of electrochemical tests in simulated FGD slurries at this pH.

Table 3 shows the results of CCT tests in a simulated anthracite slurry at pH 4. In this environment normal operating temperatures are about 500C. The results show that wrought Zeron 100 is superior to the 6%Mo wrought alloy, while the older cast 25%Cr duplex is unsuitable for this environment. Welded and cast Zeron 100 are also suitable in this application.

Table 4 shows the CCT data for a simulated lignite slurry, where operating temperatures are typically 600 - 650C. Once again wrought Zeron 100 is clearly superior to the 6%Mo austenitic alloy. Cast and welded Zeron 100 are also suitable, but the older 25%Cr duplex alloy is not.

These results show the excellent resistance of Zeron 100 to crevice corrosion, particularly the wrought product.

3.3 Erosion Corrosion

The calcium sulphate and limestone in FGD slurries are not particularly erosive, and it is the fly ash which causes most of the erosion.

Tests have been conducted in simulated FGD brines including fly ash in a recirculating erosion test rig (4). Results from pin erosion tests showed that Zeron 100 offered superior erosion resistance to austenitic alloys such as 316L as well as more highly alloyed materials. Figure 6 shows the erosion resistance of several alloys including Zeron 100 as a function of pH. It can be seen that Zeron 100 has the best erosion resistance, which is not affected by pH ie the alloy does not corrode significantly at low pH.

Following these promising results a test loop was constructed utilising a Zeron 100 pump and pipework and handling the same slurry (4). After 3000 hours running the pump and pipework were in excellent condition, with some etching of the material at the vane tips (ie the region of greatest velocity). Polishing of the impeller vanes by the slurry actually increased the pump efficiency with increasing running time.

4.0 APPLICATIONS AND EXPERIENCES

The excellent corrosion resistance of Zeron 100 combined with its high strength make it suitable for a wide range of applications in FGD plant.

Cast Zeron 100 can be used for pumps, valves and agitators, while the wrought product can be used for ducting, where acid condensation may occur, up to 3000C. Other applications include gas distribution plates, sprinkler heads, fasteners, centrifuges and absorber towers. With absorber towers the high strength of super duplex stainless steel means it is more economic to make absorber vessels from solid alloy rather than clad steel (5). One power station in the USA has changed from clad to solid super duplex for absorber vessels because of the economics (5). A recent paper (6) has described the successful operation of the absorbers for up to 20 years.

Zeron 100 has been used successfully for the slurry pumps, agitator stools and gas distribution plates at the Drax FGD plant in the UK since 1994. The pumps are lasting 30,000 to 40,000 hours between major overhauls. Zeron 100 also had the advantage that minor damage can be weld repaired, while white cast irons, as used for FGD pumps, cannot.

The slurry centrifuges at Ratcliffe FGD plant were supplied in Zeron 100 in 1996. After twelve years operation they are in excellent condition. After a short time of operation the GRP slurry return lines at Ratcliffe FGD plant were suffering severe erosion. These have now been replaced with spools in Zeron 100 and have given no further problems.

5.0 CONCLUSIONS

The combinations of high strength and good corrosion resistance make Zeron 100 eminently suitable for a wide range of applications in FGD plant. The excellent test results have lead to the use of Zeron 100 for several components in the FGD plants at Drax Power Station, UK and Ratcliffe Power Station, UK.

REFERENCES

1. R Newman, W P Wong, H Ezuber and A Garner Corrosion 45, 4 (1989) 282

2. J P Audouard, P Soulignac and F Dupoiron Materials Selection for Wet FGD Systems Presented at "Airpol '90", Louisville, USA Oct 1990

3. R Francis, WM&F Technical Note TN 1235 February 1997

4. J T Dallas and T A McConnell "Solids Pumping" Seminar organised by I Mech E London, UK Oct 1991, page 9

5. K Bendall, Paper 448 Corrosion '96, Denver, USA, March 1996. NACE

6. R.L. Richard, “20+ years of successful FGD experience with superduplex at the Gibson Generating Station”. Duplex Stainless Steel Conference, Grado, Italy, June 2007.

TABLE 1. The nominal composition of some common stainless steels.

NOMINAL COMPOSITION (WT%)

ALLOY Fe Cr Ni Mo N Cu W PREN*

316L Bal 17 10 2 - - - 24

6%Mo Bal 20 18 6 0.2 0.7 - 43 Aust. Zeron 100 (wrought) Bal 25 7 3.5 0.25 0.7 0.7 >40 Zeron 100 (cast) Bal 25 8 3.5 0.25 0.7 0.7 >40

Bal = Balance *PREN = %Cr + 3.3 x % Mo + 16 x % N

TABLE 2. Minimum mechanical properties of some stainless steels.

0.2% Proof UTS Elongn. FORM ALLOY Stress (MPa) (MPa) (%)

WROUGHT 316L 213 500 45

6%Mo Aust. 300 650 35

Zeron 100 550 750 25

CAST Zeron 100 450 700 25

TABLE 3. Critical crevice temperature for some stainless steels in a simulated anthracite FGD slurry at pH4.

Slurry composition: CaSO4 10 wt% Chloride 40 g/l Fluoride 50 mg/l “Dithionate” 200 mg/l Fe3+ 10 mg/l Al3+ 30 mg/l

FORM ALLOY CCT (°C)

Wrought Zeron 100 >80

6%Mo Aust. 64.7

Welded Zeron 100 58.5

Cast Zeron 100 57.8

Zeron 25 42.0

TABLE 4. Critical crevice temperature for some stainless steels in a simulated lignite FGD slurry at pH4.

Slurry composition: CaSO4 10 wt% Chloride 15 g/l Fluoride 200 mg/l “Dithionate” 200 mg/l Fe3+ 10 mg/l Al3+ 30 mg/l

FORM ALLOY CCT (°C)

Wrought Zeron 100 >83

6Mo aust 66.7

Welded Zeron 100 63.4

Cast Zeron 100 62.6

Zeron 25 39.6

FIGURE 1 Schematic diagram of a typical FGD absorber tower (Babcock design).

FIGURE 2 Iso-corrosion curves (0.1mm/y) for some stainless steels in sulphuric acid

110 Boiling Point Curve 100

) 80 C ° ( e r u 70 a t e r p ZERON 100

e m 60 T

50 S32750

316L 6Mo Aust. 40

30 0 10 20 30 40 50 60 70 80 90 100 Acid Concentration (wt.%)

FIGURE 3 Iso-corrosion curves (0.1mm/y) for some stainless steels in hydrochloric acid

110 Boiling Point Curve 100

) 80 C ° (

r ZERON 100 u t

a 70 r

e S32750 p m

e 60 T

50 316L 6Mo Aust. 40

30 01234567 8 Acid Concentration (wt.%)

FIGURE 4 Iso-corrosion curves (0.1mm/y) for some stainless steels in sulphuric acid plus 2g/L chloride

110 Boiling Point

100 Curve

80 ) C ° (

ZERON 100

e 70 r u t

a S32750 r

e 60 p m e T 50

40 904L 30 6Mo Aust.

20 0 10 20 30 40 50 Acid Concentration (wt.%)

FIGURE 5 Corrosion of some nickel alloys and Zeron 100 in ASTM G28A test

1 7.6mm/y 0.9

0.8

) 0.7 y

mm / 0.6 (

e t a

R 0.5 on i

s 0.4 rr o o

C 0.3

0.2

0.1

FIGURE 6 Pin erosion rig test results in a simulated FGD slurry

40 Temperature: 50°C Chloride: 5,000mg/L 35 Fly Ash: 0.1wt.% Velocity: 46m/s Austenitic 30 Cast Iron )

3 316L 25 m m (

s s

o 20 L 25%Cr Duplex e m u l 15 o

V ZERON 100

0 2 2.5 3 3.544.555.566.5 7 pH